Backport MB-49696 (Use built-in prometheus basic auth) to 7.2

Backporting....

 
We currently use a couchbase introduced basic auth in prometheus. This was needed as it wasn't supported at the time. Prometheus now supports basic auth so we should use it. This requires changes in ns_server to replace the current scramsha password with a bcrypt hashed password...and pass the info to prometheus in a file.